Facial cooling-induced bradycardia: attenuating effect of central command at exercise onset.

Facial cooling (FC) elicits a marked bradycardia at rest that appears to be reduced during exercise. This study was done to delineate the effects of exercise mediated central command from those of muscle afferent feedback and sympathetic stimulation on the attenuation of the bradycardic effect of FC during the onset of exercise. Ten healthy subjects (26 +/- 2 yr) were exposed to FC under five different conditions: 1) seated rest on the cycle ergometer, 2) onset of mild exercise (resting HR + 20 beats.min-1), 3) onset of moderate exercise (resting HR + 50 beats.min-1), 4) seated rest on the ergometer during electrical stimulation, and 5) seated rest on the ergometer during a cold immersion test (CT) (one hand immersed in an ice slurry at 0 degree C). The two exercise intensities were presumed to provide different degrees of central command. Electrical stimulation of the quadriceps was assumed to provide isolated muscle afferent feedback, while the CT served as a sympathetic stimulus. Beat-by-beat data were recorded for HR and mean arterial blood pressure for the duration of each test (50 s), and a rating of perceived pain was taken after each FC. FC elicited significant increases in mean arterial pressure during mild and moderate exercise compared with resting control (P < 0.05) and during moderate exercise compared to exercise without FC (P < 0.05). Mean decreases in HR during FC were similar for resting control (-12 +/- 3 beats.min-1), electrical stimulation (-10 +/- 3 beats.min-1), and CT (-9 +/- 3 beats.min-1). The HR response to FC during mild exercise (-7 +/- 2 beats.min-1) was significantly different (P < 0.05) from the rest condition; however, there was no significant bradycardia (-2 +/- 2 beats.min-1; P > 0.05) during onset of moderate exercise. These findings suggest that the magnitude of cold face-induced bradycardia may be attenuated at exercise onset by neural signals related to the higher levels of central motor command associated with heavier exercise.